<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div id="app"></div>
</body>
<script src="https://unpkg.com/vue@next"></script>
<script>
const app=Vue.createApp({
    mounted() {
        console.log('this.$refs.hello=',this.$refs.commonDemo);
        this.$refs.commonDemo.sayHello();
    },
    template:`
    <div ref="hello">hello Vue.js!</div>
    <common-demo ref="commonDemo" />`,

})
app.component('common-demo',{
methods: {
    sayHello(){
        alert('Hello Vue.js!')
    }
},
template:`
<div>Hello Vue.js!</div>`
})
app.mount("#app")
</script>
</html>